As is well known to those skilled in the art, it is possible to remove water from mixtures thereof with organic liquids by various techniques including absorption or distillation. These conventional processes, particularly distillation, are, however, characterized by high capital cost. In the case of distillation for example the process requires expensive distillation towers, heaters, heat exchangers (reboilers, condensers, etc.), together with a substantial amount of auxiliary equipment typified by pumps, collection vessels, vacuum generating equipment, etc.
And, as can be assumed, such operations are characterized by high operating costs, principally costs of heating and cooling--plus pumping, etc. Furthermore, the properties of the materials being separated, as is evidenced by the distillation curves, may be such that a large number of plates may be required, etc. When the material forms an azeotrope with water, additional problems may be present which for example, could require that separation be effected in a series of steps (e.g., as in two towers) or by addition of extraneous materials to the system.
There are also comparable problems which are unique to absorption systems.
It has been found to be possible to utilize membrane systems to separate mixtures of miscible liquids by pervaporation. In this process, the charge liquid is brought into contact with a membrane film; and one component of the charge liquid preferentially permeates the membrane. The permeate is then removed as a vapor from the downstream side of the film--typically by sweeping with a carrier gas or by reducing the pressure below the vapor pressure of the permeating species.
Illustrative membranes which have been employed in prior art techniques include those set forth below in Table I.
TABLE I ______________________________________ Separating Layer References ______________________________________ Nafion brand of Cabasso and Liu, perfluorosulfonic acid J. Memb. Sci. 24, 101 (1985) Sulfonated polyethylene Cabasso, Korngold, & Liu, J. Pol. Sc: Letters, 23, 57 (1985) Fluorinated Polyether USP 4,526,948 or Carboxylic Acid fluorides to Dupont as assignee of Resnickto Selemion AMV Wentzlaff blend of Asahi Glass Boddeker, and a cross-linked styrene Hattanbach butadiene (with guaternary J. Memb. Sci. 22, 333 ammonium residues on a (1985) polyvinyl chloride backing) Cellulose triacetate Wentzlaff, Boddeker & Hattanback, J. Memb Sci 22 333 (1985) Polyacrylontrile Neel, Aptel, & Clement Desalination 53, 297 (1985) Crosslinked Eur. Patent 0 096 Polyvinyl Alcohol 339 to GFT as assignee of Bruschke Poly(maleimide- Yoshikawa et. al. acrylonitrile) J. Pol. Sci. 22, 2159 (1984) Dextrine - Chem. Econ Eng. isophorodisocyanate Rev., 17, 34, (1985) ______________________________________
The cost effectiveness of a membrane is determined by the selectively and productivity. Of the membranes commercially available, an illustrative membrane of high performance is that disclosed in European Pat. No. 0 096 339A2 of GFT as assignee of Bruschke--published Dec. 21, 1983.
This reference discloses separation of water from alcohols, ethers, ketones, aldehydes, or acids by use of composite membranes. Specifically the composite includes (i) a backing typically about 120 microns in thickness, on which is positioned (ii) a microporous support layer of a polysulfone or a polyacrylonitrile of about 50 microns thickness, on which is positioned (iii) a separating layer of crosslinked polyvinyl alcohol about 2 microns in thickness.